Environmental Engineering Reference
In-Depth Information
Traditional electrical machines and power electronics are not very eficient in the low-
power generating regime, because in most conventional generating systems there is power
to spare, and consequently eficiency is less important in this low-power region. In addition,
conventional generators do not operate in this low-power region for long periods of time. On
the other hand, for wind systems, it is not critical for the generation system to be eficient in
high-wind, high-power conditions, because the rotor is spilling energy to keep the power at
the rated level. Therefore, wind systems can afford ineficiencies at high power while they
require maximum eficiency at low power—just the opposite of most other electrical generat-
ing systems.
Generators
Current commercial generator designs are either
squirrel-cage induction
or
wound-
rotor induction
, with some newer machines using the
doubly-fed induction
design for vari-
able speed. In the latter design, the turbine rotor's variable frequency electrical output is
fed into the collection system through a solid-state power converter. However, full power
conversion and permanent-magnet, low-speed synchronous machines are being introduced
in some newer turbines, because of their fault-ride-through capability, higher eficiency, and
other attributes.
Several unique designs are under development to reduce drivetrain weight and cost while
improving reliability. These have been explored in design studies under the NREL's Wind-
PACT project by Poore and Lettenmaier [2003] and Bywaters
et al.
[2004]. One approach
for improving reliability is to build a
direct-drive generator
that eliminates the complexity
of the gearbox, which is the component where most of the current reliability problems are
occurring. The tradeoff is that the slowly rotating generator must have a high
pole count
and
is therefore large in diameter. Depending on the design, a direct-drive generator can be in the
range of 4 m to 10 m in diameter and quite heavy.
The decrease in cost and increase in availability of rare-earth permanent magnets is ex-
pected to signiicantly affect the size and cost of future permanent-magnet generator designs.
Permanent-magnet designs tend to be quite compact and lightweight, with reduced electrical
losses in the windings.
An Enercon
1.5-MW direct-drive generator using rare-earth perma-
nent magnets has been studied and a prototype has been built. This design uses 56 poles and
is only 4 m in diameter, versus the 10 m for a wound rotor design [Poore and Lettenmaier
2003]. This direct-drive machine has undergone testing at NREL's National Wind Technol-
ogy Center.
Gearboxes
Converting rotor torque to electrical power in a wind turbine has historically been achieved
using a speed-increasing gearbox and an induction generator. Many current megawatt-scale
turbines use a three-stage gearbox consisting of varying arrangements of planetary gears and
parallel shafts. Fleet-wide gearbox maintenance issues and related failures with some past
designs have occurred. For this reason extensive dynamometer testing of new drivetrain con-
igurations has become standard practice, to verify their durability and reliability prior to serial
production. The life-cycle reliability of the current generation of megawatt-scale drivetrains
has not yet been fully veriied with long-term, real world operating experience. There seems to
be a broad consensus that wind turbine drivetrain technology can and will evolve signiicantly
in the next several years in order to meet future reliability and cost requirements.
A hybrid of the direct-drive approach that offers promise for future large-scale designs is
the single-stage gearbox driving a low- or medium-speed generator. This allows the use of a
generator that is signiicantly smaller than a comparable direct-drive design. The WindPACT
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